Semiconductor wafer singulation is typically accomplished using saws. However, sawing may not be suitable for cutting through advanced-materials formed on the wafer surface (i.e., low-k dielectrics). One alternative includes using lasers to remove scribe line material prior to sawing. Laser scribing, however, is confronted with a number of challenges.
Differences in ablation thresholds of materials formed on the wafer can complicate the laser ablation process. Nanosecond neodymium yttrium aluminum garnet (Nd:YAG) lasers, which are considered conventional, have a wavelength of 355 nanometers (nm). This wavelength corresponds to a photon energy of 3.5 electron volts (eV), which is higher than the bandgap of silicon (1.12 eV) and silicon carbide (2.86 eV), but less than that of silicon nitride (5.0 eV) and silicon dioxide (9.0 eV). When a laser's photon energy is less than a material's bandgap, the material is transparent to the laser. So, while conventional lasers may be capable of ablating silicon and silicon carbide, ablation of silicon nitride and silicon dioxide is more difficult.
When ablating multiple layers in a film stack, the laser's power must be set to ablate the layer with the highest ablation threshold, even if this means the power is too high for other layers. If an overlying layer is transparent, the laser's beam can transmit through it and ablate underlying layers. Underlying layer ablation can proceed explosively and thereby produce thermal and mechanical stress, damage, peeling, and cracking in the film stack, as well as generate surface particles.
Laser beams with shorter wavelengths and high photon energies can be more effective for scribing semiconductor materials than conventional lasers. However, there are a number of limitations associated with these systems. Shorter wavelength (266 nm) Nd:YAG lasers (4.7 eV photon energy) are limited by their low output power. Higher power ultraviolet (UV) lasers, such as excimer lasers (e.g., Krypton Fluoride (KrF) lasers (248 nm or 5.0 eV photon energy) and Fluorine (F2) lasers (157 nm or 7.9 eV photon energy) are limited by their low repetition rates, limited power output, and reliability. In addition, complex and costly UV optical components are needed to project and focus the shorter wavelength laser beams onto the workpiece in order to achieve the power densities needed for ablation.
Short pulse duration lasers (i.e. ultrafast lasers) can be used to reduce stress, damage, and surface particle contamination issues. Unlike conventional lasers, which ablate material by transferring energy to the materials lattice system to induce melting and evaporation, ultrafast lasers ablate by exciting the material's electrons to higher energy states (i.e. the conduction band) or even the vacuum level directly by single or multi-photon absorption (a nonlinear effect due to the extremely high electromagnetic field strength afforded by ultrafast laser systems) before energy transfer from the electronic system to the lattice system occurs. These lasers produce ultrafast bond scission, whereby lased material is removed with significantly reduced thermal and mechanical stress. However, while the overall laser scribing quality of ultrafast lasers may be superior to that of conventional lasers, they are limited by their low processing throughputs and the difficulties related to their stability, maintenance, and cost.
It will be appreciated that for simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding or analogous elements.